1. Field of the Invention
The present invention relates generally to a cylindrical elastic mount including a dynamic vibration damper, and more particularly to such a cylindrical elastic mount which can be produced with improved efficiency and is thus available at a reduced cost of manufacture.
2. Discussion of the Prior Art
A cylindrical elastic mount is known as a device for elastically or flexibly connecting two members in a vibration system in a vibration damping or isolating manner, or as a device for mounting one of the two members on the other member such that the one member is elastically or flexibly supported by the other member. Examples of such a cylindrical elastic mount are disclosed in laid-open publication No. 56-63141 of unexamined Japanese patent application, and laid-open publication No. 57-59119 of unexamined Japanese utility model application. The cylindrical elastic mount disclosed in these publications has an inner and an outer sleeve which are made of a metallic material and are disposed in radially spaced-apart, coaxial or non-coaxial relationship with each other. Between these inner and outer sleeves, there is interposed an elastic body such as a formed rubber mass, such that the two sleeves are elastically connected by the elastic body. The cylindrical elastic mount of this type is widely used as an engine mount adapted for use on a transverse front-engine front-drive (F-F) car, and as a body mount or a suspension bushing for a motor vehicle, for example.
As one type of the elastic mount as described above, there is known a cylindrical elastic mount having a dynamic vibration damper which consists of the elastic body, and a rigid member which is disposed between the inner and outer sleeves. This rigid member is embedded in or secured to the elastic body by means of vulcanization, and is supported by the inner and outer sleeves through the elastic body. In the presence of this dynamic vibration damper, the elastic mount exhibits a relatively low dynamic spring constant with respect to a middle- to high-frequency range of input vibrations. Although the rigid member included in the elastic mount may be a block-like member, it is preferable to employ a cylindrical rigid member which is disposed between the inner and outer sleeves so as to extend over the entire circumference of the mount. The use of the cylindrical member as the rigid member makes it easy to tune the dynamic spring characteristics of the elastic mount, and permits the elastic mount to provide an excellent damping effect with respect to the input vibrations applied in any radial direction.
When the cylindrical rigid member is provided between the inner and outer sleeves, the inner sleeve, the cylindrical rigid member and the outer sleeve are disposed in this order as viewed in a radially outward direction of the mount. Then, a radial spacing between the inner sleeve and the cylindrical rigid member and a radial spacing between the cylindrical rigid member and the outer sleeve are filled with respective masses of a suitable rubber material, so as to form radially inner and outer portions of the elastic body on the opposite sides of the cylindrical rigid member. In this case, when the outer sleeve is radially inwardly compressed after the elastic body is formed by vulcanization, it is practically impossible to give a suitable degree of preliminary radial compression to the radially inner portions of the elastic body interposed between the inner sleeve and the cylindrical rigid member.
In view of the above, the cylindrical rigid member is conventionally composed of radially inner and outer metallic members each having a cylindrical shape. More specifically, the conventional elastic mount consists of an inner bushing which has a first elastic body interposed between the inner sleeve and the inner metallic member for elastic connection therebetween, and an outer bushing which has a second elastic body interposed between the outer sleeve and the outer metallic member for elastic connection therebetween. After these inner and outer bushings are separately formed, the inner metallic member is radially inwardly compressed so that a suitable degree of preliminary radial compression is given to the first elastic body of the inner bushing. Similarly, the outer sleeve is radially inwardly compressed so that a suitable degree of preliminary radial compression is given to the second elastic body of the outer bushing. Thereafter, the outer metallic member of the outer bushing is fixedly fitted on the inner metallic member of the inner bushing, so as to provide an integrally formed bushing structure. In this manner, the intended cylindrical elastic mount with a vibration damper can be obtained.
In manufacturing the thus constructed cylindrical elastic mount, however, two tubular metallic members are needed for providing the cylindrical rigid member, and the inner and outer bushings must be prepared by respective vulcanizing operations for forming the first and second elastic bodies, and by respective drawing operations in which the inner metallic member and the outer sleeve are radially inwardly compressed. Further, the inner and outer bushings which are separately produced at different sites must be put together at an assembly site. Thus, the cylindrical elastic mount of the above type is manufactured with considerably low efficiency, at a relatively high cost, owing to a relatively large number of process steps required for forming the two independent bushings and assembling these bushings together. Further, this elastic mount requires two tubular metallic members, which also push up the cost of manufacture of the mount.